Glutamate (L-glutamic acid) is the major excitatory transmitter in the mammalian central nervous system, exerting its effects through both ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGluRs) belong to family C (also known as family 3) of the G-protein-coupled receptors (GPCRs). They are characterized by a seven transmembrane (7TM) α-helical domain connected via a cysteine rich-region to a large bi-lobed extracellular amino-terminal domain. While the orthosteric binding site is contained in the amino-terminal domain, currently known allosteric binding sites reside in the 7TM domain. The mGluR family comprises eight known mGluRs receptor types (designated as mGluR1 through mGluR8). Several of the receptor types are expressed as specific splice variants, e.g. mGluR5a and mGluR5b or mGluR8a, mGluR8b and mGluR8c. The family has been classified into three groups based on their structure, preferred signal transduction mechanisms, and pharmacology. Group I receptors (mGluR1 and mGluR5) are coupled to Gαq, a process that results in stimulation of phospholipase C and an increase in intracellular calcium and inositol phosphate levels. Group II receptors (mGluR2 and mGluR3) and group III receptors (mGluR4, mGluR6, mGluR7, and mGluR8) are coupled to Gαi, which leads to decreases in cyclic adenosine monophosphate (cAMP) levels. While the Group I receptors are predominately located postsynaptically and typically enhance postsynaptic signaling, the group II and III receptors are located presynaptically and typically have inhibitory effects on neurotransmitter release. Without wishing to be bound by theory, increasing evidence indicates mGluRs play an important role in lasting changes in synaptic transmission, and studies of synaptic plasticity in the Fmr1 knockout mouse have identified a connection between the fragile X phenotype and mGluR signaling.
The identification of small molecule mGluR antagonists that bind at the orthosteric site has greatly increased the understanding of the roles played by these receptors and their corresponding relation to disease. Because the majority of these antagonists were designed as analogs of glutamate, they typically lack desired characteristics for drugs targeting mGluRs such as oral bioavailability and/or distribution to the central nervous system (CNS). Moreover, because of the highly conserved nature of the glutamate binding site, most orthosteric antagonists lack selectivity among the various mGluRs.
A more recent strategy that has been able to successfully deal with the aforementioned issues has been the design of compounds that bind the mGluR at a site that is topographically distinct from the orthosteric binding site, or an allosteric binding site. Selective negative allosteric modulators (NAMs) are compounds that do not directly deactivate receptors by themselves, but decrease the affinity of a glutamate-site agonist at its extracellular N-terminal binding site. Negative allosteric modulation is thus an attractive mechanism for inhibiting appropriate physiological receptor activation. Among the most studied and characterized small molecules are the mGluR5 NAMs, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP). Both MPEP and MTEP have proven efficacious in numerous rodent models of disease, including those for drug addiction and pain as well as anxiety. The compounds were also able to inhibit transient lower esophageal sphincter relaxation (TLESD), the major cause of gastroesophageal reflux disease (GERD), in dogs and ferrets. In addition, MPEP was efficacious in mouse models of fragile X syndrome (FXS) and Parkinson's disease (PD) as well as a baboon model of binge-eating disorder.
Although the utility of MPEP and MTEP as tool compounds has been clearly demonstrated, both molecules have issues that complicate or prevent their further development as therapeutic molecules. MPEP has been shown to directly inhibit the N-methyl-D-aspartate (NMDA) receptor activity at higher concentrations and is a positive allosteric modulator of mGluR4. While these selectivity issues are mitigated with MTEP, it is a potent inhibitor of cytochrome P450 1A2 and is efficiently cleared following intravenous administration to rhesus monkeys.
Potential adverse effects of known mGluR5 NAMs, however, could reduce their ultimate therapeutic utility. Further, conventional mGluR5 receptor modulators which target the orthosteric binding site can lack satisfactory aqueous solubility, exhibit poor oral bioavailability, and/or exhibit adverse effects. Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide selective negative allosteric modulators for the mGluR5 receptor.